In this way, setHours is acting upon the value you wish to have the hours set on. This is the primary manner of using setHours.

Other details

The specification doesn't appear to mention the return value. Other sites such as w3schools do.

The Chromium setHours source shows a value being return though other functions that do similar do not return this value. I presume that the SET_LOCAL_DATE_VALUE function found chromium's date.js is assigning the value into the first argument.